1. Field of the Invention:
This invention relates generally to connection port assemblies for closed vessels. More particularly, this invention pertains to electrical heating elements and apparatus for inserting and retaining such elements in non-metallic pressure vessels.
2. Description of the Prior Art:
Electrical water heaters have been widely used in homes as well as in business places. Such water heaters comprise a metallic pressure vessel into which electrical heating elements are fixed. Typically, the pressure vessel has one or more flanged openings. The elements are fixed to a gasketed flange cover which is bolted to the flange on the pressure vessel. A leakproof seal requires that the elastomeric element between the flange cover and the vessel be highly compressed. Thus, the two are typically joined by multiple screws which are installed to provide the required high axial compression.
As is well known in the prior art, metal water heaters are susceptible to corrosion, and generally have a limited life.
The use of non-metallic materials such as plastics in the construction of water heaters has recently been established. In one exemplary water heater, the two-part vessel wall comprises an outer shell which overlays an inner liner. The shell is typically formed of a composite of a reinforcing fiber and a thermoset resin, and the liner may be formed of a thermoplastic material by blow molding or other method. The construction of ports for inlets, outlets, probes, heating elements, drains, etc. in non-metallic vessels is of special concern when the vessel is to contain liquids at elevated pressures and temperatures. Such is exemplified in the disclosure of U.S. Pat. No. 4,765,507 of Yavorsky et al. which teaches a tank port passing through a reinforcement insert .joined to the vessel wall between the shell and the liner. The fitting is swaged into a collar, and the port has internal screw threads for mounting threaded elements. The element is screwed into the port until an o-ring is sealingly compressed.
One disadvantage of such a threaded screw fitting is that elements requiring a particular rotative attitude in the port are difficult to install in the required position while simultaneously ensuring a positive seal against leakage.
In addition, salts in tile water typically become deposited on the thread surfaces, making subsequent removal of the element difficult. The high torque required to remove the threaded element necessitates extra strength to be built into the vessel wall.